In some fanfics, a plot twist could be that a demigod turns into a vampire. This completely changes the dynamic as they now have to deal with their new vampiric nature while still having their demigod powers. It's a really unique take on the crossover.
A very interesting plot twist is when the vampires discover that they are related to the Greek gods in some way. Maybe they were created by a minor god as an experiment. This throws the Vampire Diaries characters into the world of Percy Jackson's mythology and makes for a lot of drama and discovery as they try to figure out their new place in this expanded universe.
Maybe a demigod gets turned into a vampire in a strange magical accident. This would create a whole new set of challenges as they try to balance their demigod powers with their new vampire nature. They might have to deal with the vampire politics in Mystic Falls while also trying to stay true to their Olympian heritage. It could lead to some really interesting character development and power struggles.
A really interesting plot twist is when a character from Arrow gets turned into a vampire in some fanfictions. For example, in 'The Dark Transformation', Felicity Smoak gets bitten by a vampire and has to deal with her newfound vampiric nature while still trying to help Team Arrow. It completely changes her character dynamics and adds a whole new layer of complexity to the story.
In some fanfics, a plot twist could be that a vampire from Vampire Diaries turns out to be a long - lost relative of a Hogwarts student. This completely changes the dynamics as the student has to deal with their newfound family connection and the fact that they're a vampire.
In the stories, the revelation about Luke being a traitor is a big plot twist. He was initially seen as a friend and mentor to Percy. But later, it turns out he has his own dark motives and is working against the gods, which adds a lot of complexity and danger to the story.
Some fanfictions might explore how the demigods' weapons work against vampires. For instance, Percy's Riptide could potentially be very effective against vampires, as it is a magical weapon. And the vampires' speed and strength could pose a real challenge to the demigods. It would be fascinating to see how the characters from both series adapt to each other's worlds. Maybe the demigods would have to learn about the rules of the vampire world, like not being able to enter a home without an invitation, while the vampires would be shocked by the existence of gods and demigods.
One interesting plot twist could be that Percy's water powers somehow interact with Elsa's ice powers in unexpected ways. Maybe when they meet, their powers combine to create a new form of frozen water element that has unique properties. Another could be that the demigods from Percy Jackson's world have to deal with the magical cold of Arendelle, which is a whole new kind of challenge compared to their usual Greek - related threats.
Perhaps a plot twist where the Mist from Percy Jackson's world starts to affect Tokyo Ghoul. The Mist could make humans in Tokyo see ghouls as something completely different, like mythical creatures from their own Japanese folklore. This would not only change how the humans in Tokyo Ghoul react to ghouls, but also how the ghouls themselves perceive their place in the world and their relationship with humans.
One interesting plot twist could be that Percy discovers a way to nullify Kratos' god - given powers. Maybe he finds an ancient artifact during the fight that has the ability to do so. Another could be that Kratos, instead of seeing Percy as an enemy, realizes they have a common goal against a greater evil in the fanfiction world and they end up teaming up.
One interesting plot twist could be when Atalanta turns out to be a long - lost relative of one of Percy's friends. This completely changes the dynamic between them and adds a whole new layer of complexity to the story.
In some fanfics, a plot twist might be that the Death Note has some sort of connection to the Greek underworld. Maybe it was created by Hades as a way to control the souls in a different way, and when Light Yagami starts using it, he unknowingly disrupts the balance between the mortal world and the underworld. This could lead to Percy and his friends having to step in and deal with both Light and the chaos in the underworld.